Just wondering as I can't seem to find any reference, is there a magic number to which flex would be the 'easiest' based on the number in your raid. Also does the loot scale at certain thresholds which could influence this?

Some people have worked out that there are specific break points for specific mechanics of some bosses that change the makeup of the fight. It isn't consistent though from boss to boss, and so no there's no magic number.

Ultimately if you're trying to min/max Flex by not inviting more people that want to go and who would help build a successful team, you're overthinking it.

So if you're trying to maximize your game time by minimizing your time spent in a flex raid that's overthinking it? How do you feel about the rest of theorycrafting?

Some people are going to try to min/max everything because they find efficiency fun. But the intent of Flex is to offer a low stress option for guilds and friends to progress through content without rigid requirements to show up/sit out to maintain a number of participants. Similarly we like that people can PUG it and find some success in a managed group setting. Feel free to ignore the intent of the feature, but my response is still going to be that limiting yourself to some specific number of people is not required to have a successful run, and can be detrimental to the types of people and raid groups that the feature is directly intended for.

If a raid leader is attempting to build a rigid group to attempt Flex and is telling members of his own 25 Normal team to sit out because some lower number is optimal, that person is overthinking it. If someone is building a Flex PUG and is turning down skilled and geared players because of an optimal number he read somewhere, he's overthinking it. It's Flex, let's not pretend you're world first raiding here.

Right now there are some assumptions being made about mechanical breakpoints in Flex, but there are also some valid concerns. The orbs on Norushen for example may need tweaking.

What would be super helpful is if people have examples of breakpoints they think limit them to having a certain number of people, and what those are. It's in everyone's best interests to be able to invite whoever they want, and not feel like they're limited to some number because of mechanic scaling.

Thanks for some excellent feedback so far. Boss mechanics inevitably involve some breakpoints (alas, there's no way for Malkorok to create precisely 3.7 Implosions), and they do need to scale in some form, or abilities only targeting maybe 2 players out of a 25-player Flex raid would make some of those mechanics feel completely trivial and detract from the intended experience and tuning. In general, we try to err on the side of rounding down, and making sure that the ratios are never worse for any group size than they would be for a normal 10- or 25-player raid. Norushen orbs are a great example of a place where that logic doesn't quite work, though, since you actually want more orbs, and not fewer. That's something we can adjust.

Personally we have found 11 to be just about the hardest number. The difference in scaling between 10 to 11 is about 11mill HP. Now for a boss like the Iron Juggernaut, that’s fine. You pew pew the boss the whole fight. 11 mill over 5-6 mins is do-able. But, for a fight like Galakras where the boss is a burn at the end. That 11 mill is pretty tuff to make. Especially being flex so the extra members are not exceptional.

TL:DR: the boss HP Scaling is a bit over tuned for 1-2 extra players. (on some fights)

A bit of clarification here: Boss health and damage scaling are both completely linear and proportional to group size, but scale up more slowly than raid size does. So, all else being equal, if you add players who are roughly on par with the average gear/skill of your current group, you'll have a slightly easier time as a result.

Thok, for example, has 355 million health with a 10-player Flexible raid. If you add an 11th player, his health will go up to 390 million. I.e., you've increased your raid's damage output by somewhere between 14 and 20% depending on how many healers you were running, but the boss's health only went up by 10%. Adding that 11th player will also make Thok melee for about 3% more damage, which will barely be noticeable.